A police officer with a bomb-sniffing dog patrols the Times Square subway station, Friday, Sept. 9, in New York. Just days before the 10th anniversary of the Sept. 11 attacks, US counterterrorism officials are chasing a credible but unconfirmed Al Qaeda threat to use a car bomb on bridges or tunnels in New York City or Washington.

Mary Altaffer/AP

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The United States is facing a “specific, credible, but unconfirmed” threat of a potential Al Qaeda attack on US soil, likely designed to coincide with the 10th anniversary of the 9/11 terrorist attacks, according to US officials.

US intelligence specialists have picked up “lots of chatter” on jihadist websites, “and we’re taking it all seriously,” Department of Homeland Security Secretary Janet Napolitano said Thursday.

Secretary Napolitano hinted that some of the insights into this potential attack come from the cache of documents that US Special Operations Forces seized during the raid that killed Al Qaeda leader Osama bin Laden in May. “We know it’s an iconic day to Al Qaeda in part because of what was found at the [bin Laden] compound, so we are preparing accordingly,” she said.

Beyond that, Napolitano declined to provide specifics. “I don’t want to give those [details] out because I don’t want to tell the bad guys exactly what we’re doing,” she said. “But I think it’s fair to say that in addition to asking citizens to be vigilant and so forth, that we have ourselves leaned forward and have made sure that we are doing all that we can.”

Some additional information has leaked out, however, including “a possible plot directed at the homeland that seems to be focused on New York and Washington, D.C.,” a senior Obama administration official told CNN, adding that the threat is believed to involve three trained terrorists and a vehicle laden with explosives. The would-be terrorists are believed to be coming from Afghanistan, but it's not known whether they have yet entered the United States.

On the heels of the trove of documents uncovered by US Navy SEAL Team Six during their raid of Osama bin Laden’s house, senior administration officials issued a government advisory in May. The bulletin, put out by the Federal Bureau of Investigation and the Department of Homeland Security (DHS), warned that, as far back as February 2010, Al Qaeda was contemplating “an operation against trains at an unspecified location in the United States on the 10th anniversary” of the 9/11 attacks.

Al Qaeda, the warning noted, was actively investigating the possibility of “trying to tip a train by tampering with the rails so that the train would fall off the track at either a valley or a bridge.” Though the Transportation Security Administration sent a bulletin to railway officials, DHS did not issue a National Terrorism Advisory System alert.

The most recent warning Thursday indicates that Al Qaeda and its affiliates have also considered attacks with small arms, homemade explosives, and poisons.

New York CIty Mayor Michael Bloomberg on Thursday night urged residents to go about their normal routines, even as the city’s police commissioner announced that police would set up vehicle checkpoints across the city and step up bag checks of subway riders.

Early warnings that surfaced Thursday of missing rental cars in Kansas City, Mo., which some analysts were linking to the potential attack, were being downplayed Friday morning after the vehicles were recovered.

Other officials are cautioning that much remains unknown about any potential plans for an attack on US soil. Former Bush administration Homeland Security official Fran Townsend told CNN that the threat, while credible, is unverified.

There are questions and “weird things” about the information that intelligence officials have picked up, she added.

“What is ‘weird’ is that some of this [information] appears inconsistent and incompatible,” says George Lopez, an expert in counterterrorism at the University of Notre Dame, in South Bend, Ind. What’s more, terrorist groups often hype their capabilities. “The overall chatter, I suspect, is not a surprise to any agency,” he says. “There is a lot of bravado out there.”

That said, Dr. Lopez adds that officials have probably been picking up on threats that sound disturbingly similar coming from different sources of intelligence. “What may have tipped the balance” for US officials sounding the current alarm is “that there was too close a correspondent in one set of foreign chatter with the other human intel and bits of the technical data which was found in the US,” he argues. “That would – and should – draw lots of scrutiny.”